The present invention relates to a vehicle tire more particularly to an improved tread portion suitable, for low aspect ratio tires being capable of improving the wear resistance and steering stability.
Recently, low aspect ratio tires are widely used in passenger cars especially high performance cars.
In general, if the radius of curvature of the tread is increased, the steering stability during high speed straight running may be improved. But, as the tread camber decreases, and also a camber angle is given by wheel alignment, the ground pressure is increased in the shoulder portion during cornering. Accordingly, uneven wear is liable to occur in the shoulder portion. Further, the high speed durability decreases when the tire is mounted on the wheel with a relatively large camber angle.
Therefore, in order to solve these problems, a relatively large camber is given to the shoulder portion by forming the shoulder portion with an arc having a small radius of curvature of about 200 to 300 mm whereas the tread crown portion is formed by an arc having a larger radius of curvature of not less than 1000 mm. That is to say, the tread profile is formed by a double radius curve.
In such a profile, even if the two arcs, crown arc and shoulder arc are connected with each other without forming any inflection point, the ground pressure increases in the vicinity of the connecting point between the arcs, and as a result uneven wear is liable to be caused, and sometimes the steering stability during high speed straight running and cornering performance are deteriorated.
It is therefore, an object the present invention to provide a vehicle tire in which the ground pressure distribution is evened to improve the wear resistance and running performance.
According to one aspect of the present invention, a vehicle tire comprises a tread portion of which tread radius of curvature decreases substantially continuously from the tire equator to the tread edges.
Here, the tread edges are defined as the axially outermost edges of the ground contacting patch of the tire which is mounted on a standard rim and inflated to a standard pressure and loaded with a standard load. The standard rim is the xe2x80x9cStandard Rimxe2x80x9d specified in JATMA, xe2x80x9cMeasuring Rimxe2x80x9d in ETRTO, xe2x80x9cDesign Rimxe2x80x9d in TRA or the like. The standard pressure is the maximum air pressure in JATMA, xe2x80x9cInflation Pressurexe2x80x9d in ETRTO, maximum pressure given in the xe2x80x9cTire Load Limits at Various Cold Inflation Pressuresxe2x80x9d table in TRA or the like. The standard load is 88% of the maximum load, which is the maximum load capacity in JATMA, xe2x80x9cLoad Capacityxe2x80x9d in ETRTO, maximum value given in the above-mentioned table in TRA or the like.
Preferably, the tread profile is defined by a curve which is a part of the locus of an equation such as elliptic equation, cycloid equation, epicycloid equation, involute equation and the like, which equation is differentiatable in the range of variables corresponding to the range from the tire equator to each tread edge.